1. Field of the Invention
This invention relates to a body coated with cubic boron nitride, and a method for manufacturing the same.
2. Description of the Prior Art
Recently a film-making technique has been remarkably developed. Various coating techniques of the cubic boron nitride have been developed for coating a part, a tool or the like which requires corrosion-resistance, wear-resistance and high hardness.
Methods disclosed in the Japanese Patent Opening Gazettes Nos. 204370/1986, 47472/1987 and 77454/1987 are given as examples for the coating techniques.
In the method disclosed in the Japanese Patent Opening Gazette No. 204370/1986, prasma abundant in electrons is produced by hollow cathode discharge, and a part of the electrons are attracted towards a reaction gas inlet. Gas is activated, and so reactivity for physical vapour deposition has been improved.
In the method disclosed in the Japanese Patent Opening Gazette No. 47472/1987, a DC or AC bias voltage is applied to a reaction gas introducing nozzle which is called "activation nozzle", so that plasma is produced with high density. Ions are injected to a body to be coated, from the plasma of high density. Radio frequency (rf) bias voltage is applied to the body to be coated. Thus, a cubic boron nitride film is formed on the body.
In the Japanese Patent Opening Gazette No. 77454/1987, such a method for forming a cubic boron nitride film is disclosed that a DC or AC bias voltage is applied to the activation nozzle to produce a plasma of high density, a rf bias voltage is applied to a body to be coated, and a reaction gas such as Nitrogen gas or hydronitrogen gas and a discharge base gas such as Argon are mixed and introduced into the vacuum chamber through the activation nozzle, or concurrently introduced thereinto.
In both of the above described second method (Japanese Patent Opening Gazette No. 47472/1987) and third method (Japanese Patent Opening Gazette No. 77454/1987), the bias voltage is applied to the gas introducing nozzle so that plasma of high density is produced adjacent to the opened portion of the gas introducing nozzle, while the rf bias voltage is applied to the body to be coated, and ions are injected into the body from the plasma of high density so as to form a cubic boron-nitride film on the body.
Besides the above-described methods, a sputtering method, an ion-beam deposition method and an ion plating method are well known for forming a cubic boron nitride film.
The X-ray diffraction method is used for judging the structure of the film. It was reported that the cubic boron nitride film was judged to be formed by the fact that only one peak was obtained nearly at the diffraction angle 2.theta.=43.degree.within the range of 20.degree. to 50.degree. as shown in FIG. 1.
However, the structure of the boron nitride film which was judged to be cubic-crystalline by the fact that the maximum peak was obtained nearly at the diffraction angle 2.theta.= 43.degree. according to the X-ray diffraction method, is classified into two kinds according to the infrared absorption spectrum method. One of the two kinds shows the absorption peaks at the wave numbers of about 1400cm.sup.-1 and 800cm.sup.-1 , as shown in FIG. 2. The other of the two kinds shows the absorption peak at the wave number of about 1050 cm.sup.-1. The Vickers hardness of the film of the latter characteristic is 5000 to 6000 kg/mm.sup.2. The base material coated with such a film, which requires high wear resistance and high hardness, can have a further longer life.
Hitherto, the boron nitride films which were judged to be cubic-crystalline by the X-ray diffraction method, show the absorption peaks at the wave numbers of about 1400 cm.sup.-1 and 800cm.sup.-1 according to the measurement of the X-ray absorption spectrum. Their Vickers hardness is 2000 to 4000 kg/cm.sup.2. Accordingly, it is considered that they include essentially graphite structure (hexagonal boron nitride). They cannot be judged to be cubic-crystalline from the measurement results of the infrared absorption spectrum.
On the other hand, the boron nitride film formed by the method disclosed in the Japanese Patent Opening Gazette No. 47472/1987 or 77454/1987 was confirmed to be cubic-crystalline both by the X-ray diffraction measurement and infrared absorption spectrum method.
However, the film of a few hundreds A thick near the surface boundary has graphite structure (h-BN film) which shows the absorption peaks at the wave-numbers of about 1400 cm.sup.-1 and 800cm.sup.-1 according to the infrared absorption spectrum measurement, even in the base material coated with the cubic boron nitride film which shows the absorption peak at the wave number of about 1050cm.sup.-1. The surface boundary between the base material and the film is instable to moisture in the atmosphere. Adherence strength between them is essentially low. Accordingly, when the base material coated with such a film is let alone in the atmosphere, it has been found that the film is easily peeled from the base material due to the internal stress of the film.
On the other hand, the boron nitride film containing excess boron (B/N&gt;1) has graphite structure according to the infrared absorption spectrum measurement. However, the film strength can be improved, since B--B bonds exist therein. When such a film containing excess boron is interposed as an interlayer between the cubic boron-nitride film and the base material, the adhering strength of the film can be improved. However, when the thickness of the cubic boron nitride film is increased for a practical use, the peeling occurs at the surface boundary between the boron nitride film containing excess boron, and the cubic boron nitride film. Accordingly, it cannot be practically used.